<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>

<!--  margin:0 auto;居中
  要求：块元素，块元素有固定的宽度
  -->
    <style>
        div{
            width: 100px;
            height: 100px;
            border: 10px solid red;
            box-shadow: 10px 10px 100px yellow;
        }
        img{
            border-radius: 50px;
            box-shadow: 10px 10px 100px yellow;
        }
    </style>

</head>
<body>
<div></div>

<div style="width: 500px;display: block;text-align: center">
    <img src="images/a.jpg" alt="">
</div>

</body>
</html>